Many commercial and research sectors have a need for rapid three-dimensional (3D) measurements of objects. Recently, 3D range cameras (e.g., laser range cameras (LRC), range imaging cameras, range cameras, 3D cameras, time-of-flight cameras, ToF cameras) have gained in popularity due to certain advantages over other types of 3D dimensioning systems such as laser scanners (e.g., LIDAR), and due to advances in technology making the use of 3D range cameras more practical. Range imaging cameras resolve distance based on the known speed of light using a time-of-flight technique. An illumination unit such as a laser or LED array illuminates the field of view. The reflected light is gathered by optics onto an image sensor (e.g., CCD, CMOS). Each collector (e.g., pixel) of the image sensor simultaneously measures the time that it took for the light to travel from the illumination unit to the target object and back to the range imaging camera.
A principal advantage of range imaging cameras is that they are typically able to resolve distances much quicker than laser scanning systems such as LIDAR. Their speed is principally attributable to the fact that the range imaging camera calculates distances to each point in parallel, whereas laser scanning techniques measure distances point by point as the laser passes over the entire target object. Because each pixel detects the distance to its corresponding point on the target object simultaneously, the range imaging camera is able to capture complete images very quickly (e.g., about 100 frames per second). The high-speed nature of range imaging cameras makes them well-suited for real-time applications. For example, range imaging cameras have been used experimentally to control driverless automobiles, and are used to enable certain robotic devices to maneuver through their environment. Another advantage enjoyed by range imaging cameras is that they afford greater simplicity and durability due to their lack of moving parts. In contrast, laser scanning devices typically employ a rotatable mirror to sweep the laser across the target object. Range imaging cameras also tend to be less expensive.
Although there is tremendous potential for range imaging devices in a variety of commercial and research sectors, there remain challenges to the reliability, and therefore usability, of the technology. For example, external factors that interfere with the detection of light reflected back to the range imaging camera can contribute to errors in distance measurement. Background light (e.g., ambient light) can reach the pixels, thereby increasing the signal-to-noise ratio and diminishing the ability of the pixel to obtain an accurate determination of the light beam's travel time. Similarly, interference problems can result when multiple range imaging cameras are in use at the same time, which can lead to one camera detecting the reflected signal generated by the other camera. Other sources of distance detection error for these types of systems may include pixel saturation, mixed pixels, motion artifacts, and internal scattering (e.g., internal reflections of the received signal between the gathering lens and image sensor). Systemic distance measurement errors can result in greatly reduced distance measuring accuracy (e.g., errors of up to tens of centimeters).
Range image cameras are not the only types of dimension imaging devices susceptible to errors in measurement. The aforementioned laser scanners can also experience errors that bring their measurements outside of accepted tolerances. Because the error correction techniques discussed herein could be applied to any of the various types of dimensioning cameras, the term “3D scanner,” as it is used in this disclosure, is intended to broadly encompass any type of imaging device adapted to measure the dimensions of an object, including range image cameras, laser range cameras (LRC), range imaging cameras, range cameras, 3D cameras, time-of-flight cameras, ToF cameras, Lidar, stereo imaging cameras, and triangulation range finders.
Therefore, there exists a need for a method of correcting measurement errors in a 3D scanner.